1. Field of the Invention
This invention relates to a method of making colored magnetic particles and to these colored magnetic particles as compositions-of-matter and to their use in electrostatic toner compositions.
2. Description of Related Art
Electrostatic charge patterns may be reproduced by means of one of the generally known electrostatographic printing processes, e.g., xerography or by means of a stylus as used for example in a computer printout. The resulting charge pattern may be made visible by means of a toner powder, which by one of the many conventional methods known, is brought into contact with the charge pattern to be developed. These toner powders generally consist of finely divided particles containing a binder and coloring agents.
For some electrostatographic printing applications, it is desirable that the toner powders also contain a magnetic material.
Typical magnetic materials which have appropriate magnetic and electrical properties for use in the preparation of such toner powders include finely divided metal powders of iron, nickel, cobalt, chromium dioxide, gamma ferrioxide and ferrites having a particle size in the range of from about 1 to 50 microns. These materials are, however, dark or black in color which means that they are suitable only for the production of dark or black toner images. Such magnetic particles can not be satisfactorily employed in color electrophotography wherein full color images are produced by the color separation technique using cyan, magenta and yellow colored toners, since obviously the dark color of the magnetic material will have an adverse affect on the quality of the color image.
The main approach in the prior art for solving the color problem inherent with the use of magnetic particles has been to form a coating of a pigment or dyestuff having the appropriate color on the surface of the magnetic particle or on the surface of the toner particle which contains magnetic particles dispersed in a resin binder. For example, Heikens et al (U.S. Pat. No. 4,443,527) discloses the preparation of colored toner particles containing magnetic material wherein a magnetic particle or a toner particle containing a mixture of finely divided magnetic particles dispersed in a fusible binder is first coated with a masking layer composed of a reflecting pigment such as titanium dioxide dispersed in a binder resin, followed by contact of the masked particle with a suitable dye or pigment composition wherein the dye or pigment is caused to coat or become embedded in said masking layer. However, colored renditions produced using this toner material have the disadvantage that their brightness and, in some cases, their color saturation is relatively low, probably as a consequence of the smearing of the toner particles during heat fusion of the image to the transfer substrate. A similar approach is disclosed in Bakker et al (U.S. Pat. No. 4,623,602), except that the masking layer and colored layer contain a yellow fluorescent dye, and binders are used in which dye fluoresces.
Mehl (U.S. Pat. No. 4,536,462) discloses colored magnetic toner powder comprising essentially spherical toner particles containing ferromagnetic particles, highly conductive carbon, at least 5% by weight of a sublimable dyestuff and at least 4% by weight of a surface active agent, wherein the toner particles are preferably structured to have an inside nucleus zone containing the dyestuff and surfactant, and an outside shell zone containing the magnetic material and the highly conductive carbon.
Other approaches to preparing colored toners containing magnetic particles are disclosed in Maekawa et al (U.S. Pat. No. 4,530,893) wherein toner particles are prepared from a mixture of magnetic powder, a red azo pigment and a binder resin in and Hosfeld et al (U.S. Pat. No. 4,486,523) wherein magnetic particles are coated with finely divided opaque polymeric particles which are further mixed with dyes or pigments.
Although these and other prior art approaches for providing colored magnetic toner materials are effective to varying degrees, it is often the case that the color rendition produced by the electrostatographic process is not totally satisfactory, This can arise as a consequence of slight smearing of the toned image thereby exposing portions of the original dark color of the magnetic particles. Also, the application of resinous pigmented coatings to the surface of the magnetic or toner particles can alter the magnetic properties of the toner, and lead to an increase in the electrical resistivity of the toner which can be a negative factor in many applications.